1. Field of the Invention
The present invention relates to a switch and a bridged network, and in particular to a switch supporting a multiple spanning tree protocol and a bridged network formed of such a switch.
2. Description of the Related Art
A bridged network, as shown in FIG. 1A for example, is formed of switches B1–B5 provided with a bridge function for switching a transmission frame according to a MAC (Media Access Control) destination address, terminals A11–A13 connected to the switch B1, terminals A31–A33 connected to the switch B3, terminals A41–A43 connected to the switch B4, and terminals A51–A53 connected to the switch B5. The switches B1–B5 form loops. A switch and a terminal as well as a switch and another switch are connected with an LAN (Local Area Network).
In such a bridged network, a frame is relayed along a communication path (spanning tree) formed by a spanning tree protocol (STP) standardized by IEEE802.1D.
In an example of the bridged network in FIG. 1A, a spanning tree such as shown by thick lines in FIG. 1B is formed. Since a frame is always relayed through the switch B2 in this case, the switch B2 is called a root switch.
When such a spanning tree exists solely on a bridged network, it is called a single spanning tree (SST). In this single spanning tree, each switch has only to relay the received frame according to the sole spanning tree.
Thereafter, a VLAN (Virtual Local Area Network) technology has been utilized in the bridged network, whereby it has become possible for the switch to distinguish traffic. It is not economical that unused paths exist although the single spanning tree has redundant paths. Therefore, a multiple spanning tree protocol (MSTP) is standardized by IEEE 802.1 s for the purpose of taking advantage of a plurality of paths by forming a spanning tree per VLAN.
Namely, if all of the switches B1–B5 in the bridged network example shown in FIG. 1A support the multiple spanning tree protocol, five spanning tree protocols in total are executed including not only the spanning tree shown in FIG. 1B but also the spanning tree shown in FIG. 1C with the switch B1 as a root switch, thereby forming the spanning trees according to the respective settings of the spanning tree protocols.
The frame is relayed by using any one of the spanning trees. Which spanning tree is used for the communication is determined depending on the VLAN to which the frame belongs.
Thus, in the multiple spanning tree protocol, the VLAN to which the received frame belongs is determined, and the frame is relayed according to the spanning tree to which the VLAN is allocated. Therefore, the multiple spanning tree protocol has been used only in the VLAN environment, and has uniquely been determined by the VLAN.
As a result, there has been a problem that even if there is an optimum spanning tree (for example, a spanning tree with a smaller hop count, etc.), the said spanning tree cannot be used.